U.S. patent number 5,233,190 [Application Number 07/797,462] was granted by the patent office on 1993-08-03 for fourier transform molecular spectrometer.
This patent grant is currently assigned to Leybold Inficon Inc.. Invention is credited to Duane P. Littlejohn, James E. Phillips, Fritz H. Schlereth.
United States Patent |
5,233,190 |
Schlereth , et al. |
* August 3, 1993 |
Fourier transform molecular spectrometer
Abstract
A molecular spectrometer is provided that performs Fourier
analysis utilizing the discrete Fourier Transform on a digitized
time domain waveform that relates to the composition of a sample.
Digitized reference waveforms are employed to permit the instrument
to limit its analysis to frequencies of interest and thereby
increase the rapidity of the analysis. Data at differing
frequencies can be resolved at independent resolutions, and the
instrument can analyze spectroscopic data in real time.
Inventors: |
Schlereth; Fritz H. (Syracuse,
NY), Littlejohn; Duane P. (Manlius, NY), Phillips; James
E. (Brookline, MA) |
Assignee: |
Leybold Inficon Inc. (East
Syracuse, NY)
|
[*] Notice: |
The portion of the term of this patent
subsequent to October 1, 2008 has been disclaimed. |
Family
ID: |
27051453 |
Appl.
No.: |
07/797,462 |
Filed: |
November 22, 1991 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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494580 |
Mar 16, 1990 |
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Current U.S.
Class: |
250/291; 250/282;
250/283 |
Current CPC
Class: |
H01J
49/38 (20130101); G01R 33/64 (20130101) |
Current International
Class: |
G01R
33/64 (20060101); H01J 49/34 (20060101); H01J
49/38 (20060101); G01D (); H01J 049/00 () |
Field of
Search: |
;250/282,283,291 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Anderson; Bruce C.
Attorney, Agent or Firm: Wall and Roehrig
Parent Case Text
This application is a continuation-in-part of application No.
07/494,580 for FOURIER TRANSFORM MOLECULAR SPECTROMETER filed Mar.
16, 1990, now abandoned.
Claims
What is claimed is:
1. A method of mass spectroscopy, comprising the steps of:
placing a sample to be tested into a cyclotron cell;
reducing atmospheric pressure within said cyclotron cell;
establishing a magnetic field within said cyclotron cell;
exciting coherent motion of charged particles in said sample so
that said charged particles orbit within said cyclotron cell at
frequencies that are characteristic of said particles;
converting orbital motion of said excited particles into a time
domain analog waveform that is a summation of signals that are
generated by said excited particles;
digitizing said analog waveform to provide a digital output
waveform;
generating at least one digitized reference waveform having a
frequency of interest that is characteristic of orbital motion of a
given charged particle;
multiplying point by point the digital output waveform and said
digitized reference waveform to obtain resultant products, and
summing the resultant products to provide a data point signal
having information contained therein that is determinative of said
given charged particle,
whereby following said steps of generating, multiplying and summing
a Discrete Fourier Transform (DFT) that is limited to said
frequencies of interest is accomplished on said digital output
waveform.
2. The method of claim 1, wherein said steps of converting,
digitizing, generating, multiplying and summing are performed
simultaneously, so that a DFT is performed in real time during
spectroscopy.
3. The method of claim 1, further including the step of associating
a portion of said digital output waveform with a digitized
reference waveform prior to said step of multiplying, and said step
of multiplying is performed with a plurality of digitized reference
waveforms, so that a DFT is performed at an independent resolution
on said digital output waveform with each of said digitized
reference waveforms.
4. The method of claim 1, further comprising the step of ionizing
particles that are contained within said sample.
5. The method of claim 1 further including the steps of:
storing data relating to the digital output waveform in a
memory;
performing a Fast Fourier Transform on the digitized waveform data
that is stored in said memory; and
identifying said frequency of interest from said Fast Fourier
Transform.
6. The method of claim 1 that includes the further step of storing
data relating to said digital output waveform in a memory along
with a plurality of preselected digital reference waveforms and
performing a Discrete Fourier Transform on said digital output
waveform that is limited to each of said stored reference
waveforms.
7. The method of claim 1 that includes the further step of
filtering said analog waveform prior to digitizing said analog
waveform to remove unwanted noise therefrom.
8. The method of claim 1 that includes the further step of
processing a plurality of different digitized reference waveforms
through a single multiplier on a time shared basis.
9. The method of claim 1 that further includes the step of applying
the data point signal to a visual readout means for immediate
evaluation.
10. The method of claim 1 wherein said step of digitizing includes
the further step of selecting a number of bits to represent a data
sample at a desired resolution.
11. A mass spectrometer having a cyclotron cell that includes
evacuation means connected to said cell for reducing atmospheric
pressure in said cell to a predetermined level;
sample introduction means for placing a sample to be analyzed into
said cell;
means for establishing a magnetic field in said cell;
sensor means for detecting orbiting charged particles that are
subjected to electromagnetic fields within said cyclotron cell and
for converting orbital motions of said charged particles into a
time domain analog waveform that is a summation of signals
generated by said orbiting charged particles;
digitizing means, coupled to said sensor means, for converting said
analog waveform to a digital output waveform;
means for generating a digitized reference waveform having a
preselected frequency of interest that is characteristic of orbital
motion of a given charged particle;
multiplying means, coupled to said digitizing means, for
multiplying said digital output waveform point by point with said
digitized reference waveform to obtain resultant products; and
adder means, coupled to said multiplying means for summing the
products to provide a data point signal,
whereby said means for generating, said multiplying means and said
adder means cooperate to accomplish a Discrete Fourier Transform
that is limited to frequencies of interest on said digital output
waveform, and said data point signal has information contained
therein that is determinative of said given species of charged
particles.
12. The apparatus of claim 11, further comprising means for
ionizing particles in said sample.
13. The apparatus of claim 11, wherein said sensor means, said
digitizing means, said means for generating a digitized reference
waveform, said multiplying means, and adder means operate
simultaneously, so that a discrete Fourier transform is performed
in real time.
14. The apparatus of claim 11, further including means, coupled
with said means for generating a digitized reference waveform and
said multiplying means, for associating a portion of said digital
output waveform with a digitized reference waveform, so that a
given resolution at a frequency of interest can be achieved.
Description
BACKGROUND OF THE INVENTION
This invention relates to spectroscopy wherein specific frequency
signals related to physical events of interest occur simultaneously
with other frequency signals and, in particular, to the use of a
Discrete Fourier Transform in spectroscopy or other similar
analyzing techniques wherein a plurality of simultaneously
occurring physical events manifest themselves in the form of
periodic oscillations.
As one example of the spectroscopies exhibiting these features,
there is disclosed in U.S. Pat. No. 3,937,955 to Comisarow et al, a
multi-channel ion cyclotron resonance (ICR) mass spectrometer that
utilizes Fast Fourier Transform (FFT) to analyze an entire range of
frequencies. A gas sample is introduced into the ICR cell where
molecular species contained in a sample are ionized and then
excited whereby the ions orbit at different frequencies determined
by their mass. The image current produced by the orbiting ions is
sensed and a waveform generated that contains information relating
to the species present in the sample. The waveform data is
digitized and the digitized information transformed using a Fast
Fourier Transform (FFT) operation.
The use of FFT in an ICR instrument offers rapid means for
analyzing various types of samples and is an effective method by
which the entire spectrum within the range of the instrument can be
examined. The characteristics of FFT are, however, not truly
compatible with the ICR sensor because the sensor frequencies are
inversely related to the mass of ions present in a sample, while
the FFT analysis occurs at fixed frequency intervals and requires
that all frequencies within the spectrum be analyzed. As a
practical result, much of the available instrument computer power
is expended on analyzing segments of the spectrum that contains
either no information at all or information that is of no
analytical interest to the user because the frequency or
frequencies of interest occur only in a small segment of the
overall spectrum.
It should be further noted that the resolution of a FFT instrument
is also limited by the amount of time and computer power available
to resolve all frequencies within the instrument's spectral range
without regard to what the spectrum contains or, more importantly,
does not contain. This again places a heavy demand on the
instrument in terms of time and power and, as a consequence, the
cost of building and operating a high resolution instrument is
typically high.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to rapidly and
efficiently detect specific physical events relating to chemical
spectroscopy that manifest themselves in the form of periodic
oscillations in the presence of other similar events.
It is a further object of the present invention to utilize a
Discrete Fourier Transform in chemical spectroscopy for analyzing
discrete frequency signals contained within a broad frequency
range, containing other signals.
Another object of the present invention is to improve spectrometry
by high resolution analysis of discrete frequency signals of
specific interest.
Yet another object of the present invention is to provide a
spectrometer that utilizes Discrete Fourier Transform to detect
discrete frequencies to the exclusion of other frequencies to more
effectively employ available computer power.
A still further object is to combine the advantages of Fast Fourier
Transform with those of Discrete Fourier Transfer when analyzing
spectrometry data.
These and other objects of the present invention are attained by
means of an instrument for generating a time domain waveform
containing a mixture of periodic oscillations, each of which
relates to the frequency of a physical event. A Discrete Fourier
Transform of the time domain waveform is achieved by multiplying
the waveform with at least one digitized reference waveform point
by point and summing the resultant products. The magnitude of the
resultant sum indicates the presence of the physical event
occurring at the same characteristic frequency as the digitized
reference frequency. In one embodiment of the invention a low
resolution Fast Fourier Transform operation is performed on the
digital output signal to identify spectral regions of specific
interest on which to perform a subsequent Discrete Fourier
Transform.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of these and other objects of the
present invention, reference shall be made to the following
detailed description of the invention which is to be read in
association with the accompanying drawings, wherein:
FIG. 1 is a schematic block diagram showing an instrument embodying
the teachings of the present invention;
FIG. 2 is a block diagram showing a portion of the signal
processing circuitry employed in the instrument of FIG. 1;
FIG. 3 is a diagrammatic view showing a plurality of time domain
signals sensed by the instrument shown in FIG. 1, along with a
composite waveform generated by the instrument sensor which is a
summation of the sensed signals;
FIG. 4 is a diagrammatic view showing a Fast Fourier Transform of
the composite waveform shown in FIG. 3 wherein signal amplitude is
plotted against frequency;
FIG. 5 is an enlarged diagrammatic view similar to that shown in
FIG. 4 illustrating a Discrete Fourier Transformation at a first
resolution; and
FIG. 6 is an enlarged diagrammatic view similar to that shown in
FIG. 4 further illustrating a Discrete Fourier Transformation at a
second higher resolution.
DETAILED DESCRIPTION OF THE INVENTION
Although the present invention has broader application, it will be
herein explained with reference to an ion cyclotron resonance (ICR)
mass spectrometer. As will become apparent from the disclosure
below, however, the invention is ideally suited for use in any type
of instrument used to detect or analyze data relating to
simultaneously occurring physical events that manifest themselves
as periodic oscillations. As disclosed by Comisarow et al. in the
above noted U.S. patent, ICR mass spectrometers have been greatly
improved through the use of Fast Fourier Transform techniques.
These instruments, nevertheless, have practical limitations based
on the cost of equipment, the amount of computer power consumed,
and the processing time required to analyze the acquired data.
With reference to the drawings, and initially to FIG. 1, there is
shown an ICR ionization cell generally referenced 10, which is in
the form of a "pillbox" having opposed rectangular walls made of an
electrically conductive material. The cyclotron cell is shown
mounted in a vacuum chamber that is designated 11. The cell has
opposed top and bottom plates, a pair of opposed side plates and a
pair of opposed end plates. Small apertures, such as aperture 12,
are provided in axial alignment in the two end plates to allow an
electron beam emanating from electron gun 13 to pass through the
cell along axis 14 to ionize molecules of a sample contained within
the cell.
A trapping potential is carried by leads 15--15 to the end plates
of the cell while an excitation potential is applied by leads
16--16 to the top and bottom plates thereof. The two side plates
are used to sense or detect image current data and send this
information to a preamplifier unit 26 via leads 17--17. A strong
magnetic field is placed uniformly over the cell with the lines of
flux extending parallel with the axis 14 of the ionizing beam. The
chamber 11, during operation, is evacuated and a sample to be
analyzed is admitted into the chamber through pressure reducing
inlet (not shown). The sample typically contains one or more
molecular species of interest which, at low pressures, become
randomly distributed throughout the chamber and the cyclotron cell
10.
The operation of the cyclotron cell is well known and is discussed
in detail in the previously noted Comisarow et al. patent. The
disclosure contained in the Comisarow et al. patent is herein
incorporated by reference to the extent needed for a more thorough
understanding of the present invention. The sample within the cell
is subjected to an ionizing process. This can be done by momentary
exposure to an electron beam causing some of the sample molecules
to become ionized. Other suitable methods of ionizing a sample may
be used as required by a given application, such as chemical or
photo-ionization, or collisional dissociation. The ionizing step
can be omitted if the sample to be analyzed is already in ionic
form. The ionized molecules orbit within the magnetic field about
the axis 14. The size of each individual orbit is determined by the
thermal energy of the ion and the mass of the molecule contained in
the orbit. The trapping potential on the end plates is raised to a
level so as to contain the ions within the cell for a finite period
of time. The ions thus experience a simple harmonic motion along
the axis of the cell, while maintaining their particular orbital
motion around the axis, with each molecular species orbiting at its
own cyclotron frequency. The ions are then excited into higher
orbital state by momentarily applying a sinusoidal signal of the
same frequency as its cyclotron frequency to the top and bottom
excitation plates. The electrical charges produced by the rotating
ions are detected by the two opposed side plates and the output is
applied to leads 17--17 to provide a time domain waveform that is a
summation of the sinusoidal image currents generated by the
orbiting ionized molecules contained within the magnetic field
established inside the cell.
Turning now to FIGS. 3-6, it will be assumed for purpose of
explanation that four discrete molecular species are present in a
gas sample introduced in the ICR cell 10 shown in FIG. 1
Accordingly, four frequency signals are detected by the sensing
plates of the cell which are illustrated as the sinusoidal
frequency signals f1-f4 in FIG. 3. These four frequency signals are
contained in the output waveform generated by the sensing plates of
the cell. The waveform is illustrated by the representative curve
21 in FIG. 3.
As illustrated in FIG. 1, the present system utilizes a computer 24
located at a user work station to interface with various system
components. The computer issues high level commands as well as
providing a visual display of the resulting analysis. The computer
is connected by appropriate leads to a sensor control electronic
module 25 which generates necessary control signals and excitation
voltages that are applied to the plates of ionization cell 10.
The composite output waveform 21 (FIG. 3) produced by the cell is a
time domain signal which is initially sent through a preamplifier
26 to a post amplifier anti-aliasing filter 27 where unwanted
background and noise are removed from the amplified waveform. The
filter is controlled by a signal from the sensor control module 25.
The filtered time domain waveform is then applied to an analog to
digital converter 28 where waveform data is digitized. A digital
output waveform, comprising a stream of digital bytes, each having
a sample width containing a predetermined number of bits, is fed to
a control processor 29. The control processor is coupled to the
computer by bus 30 and to a signal processor 31 and a memory unit
32 by a bifurcated bus 33.
The signal processor of the present instrument contains circuitry
that is able to perform both a low resolution (or even high
resolution) FFT and a high resolution DFT computations on the
digitized waveform data stored in the waveform section 41 of the
memory (FIG. 2). The low resolution FFT computation is used to
provide an initial presentation on the computer screen of the
entire frequency spectrum present in the output waveform as
illustrated in FIG. 4. This, in turn, allows for rapid
identification of those frequencies that are present relating to
specific molecular species of interest without expending a great
deal of computer power. With this information, the instrument can
be programmed to perform DFT computations on preselected spectral
segments or heuristically determined segments based on
preprogrammed selection criteria. This is accomplished by first
loading data relating to one or more reference signals relating to
the frequencies of interest into the memory. The control processor
includes a tunable reference signal generator which, upon command
from the computer, places the desired reference signal of a
selected frequency into the reference signal section 42 of the
memory. The sinusoidal reference wave or waves are selected to
match the cyclotron frequency or frequencies of ionic species of
specific interest.
As illustrated in FIG. 2, the waveform section 41 of the memory is
connected to a first input 45 of a multiplier circuit 46 while the
reference signal section 42 of the memory is connected to the
second input 47 of the multiplier circuit. The output of the
multiplier circuit is coupled to adder circuit 50 that is wired in
and includes an accumulator. In practice, the reference signals can
be either stored in memory as shown, or fed directly to the
multiplier from the computer through the control processor 29.
Although a multiplier is utilized in the preferred embodiment of
the present invention, other circuits suitable for combining and/or
comparing the output waveform with a reference signal of a
predetermined frequency to determine if there are components in the
waveform that are at the reference signal frequency can be used
without departing from the teachings of the invention.
The stored bytes relating to the digital waveform data and
corresponding digital reference frequency signal data are fed to
the multiplier circuits where they are multiplied point by point
and the resultant products summed by the adder and accumulated to
provide a data point signal. As long as the reference signal is
coherent with some component of the output waveform, the adder will
continue to sum the resultant products and provide a data point
signal. If the reference signal frequency is not present in the
digitized waveform, the resultant product values will cancel each
other and the output of the adder circuit will be near zero. The
output resolution of the adder circuit can be enhanced by either
increasing the data sampling times or by increasing the number of
bits in each byte used to represent a data sample.
The signal envelope of the adder circuit is illustrated by curve 49
shown in FIG. 5. In this particular case, the frequency of the
reference signal is centered at about frequency fl and the
resolution of transform is low. As a result, ionic species
represented by frequencies f1 and f2 are included beneath the
signal envelope. To more clearly resolve the data point at
frequency f1, the summing time of the adder circuit can be
increased and/or the number of bytes relating to the digital
waveform can be increased. This produces a sharper resolution
envelope as shown at 50 in FIG. 6. In this case the resolution is
high enough to exclude the data point signal relating to frequency
f2. Data relating to frequency f1 is clearly discernable and thus
can be accurately analyzed.
The DFT processor 40, as shown in FIG. 2 can be realized using a
digital processing circuit such as that shown in model AD 2100 from
Analog Devices Corporation. A single circuit as this can be
operated in a time division multiplex mode to analyze several
frequencies simultaneously. Under normal operating conditions, a
device having the clock rate of the AD 2100 can be used to
construct 50 DFT filters when operated in a time multiplexed mode
which, in turn, can be used to analyze a like number of discrete
frequencies at the same time.
The advantages of the invention as presented above can be further
appreciated by a discussion of certain attributes of Fourier
Processing and an example: As is well known, the FFT represents a
fast algorithm for the computation of the discrete Fourier
Transform (DFT). However in order to take advantage of the FFT it
is necessary to operate on a batch of data. When variable frequency
resolution is required, this is a serious drawback, because each
batch must be processed at a fixed resolution. And, as previously
mentioned, the necessary processing of frequencies of no interest
by the FFT is wasteful in computation time, especially in those
instances where it is known in advance that there are only a few
frequencies of interest in the spectrum.
Typical FFT analysis of mass spectrometer data proceeds in
accordance with the following procedure:
1. Collect and store a time data set, say 1024 points. It is not
necessary, but is often convenient to use a number of points that
is a power of 2.
2. Perform the FFT on this data set. Resolution is then determined
by the time the spectrometer is sensing a sample containing charged
particles, or the measurement time T.sub.w. The resolution is
approximately proportional to 1/T.sub.w. It will be apparent that a
higher resolution requires a longer measurement time. To accomplish
the FFT it is necessary to compute the energy at all frequencies,
regardless of whether it is needed.
The use of the DFT, while requiring more computations, has the
advantages that it can single out particular frequencies for
analysis and each of the frequencies can be analyzed at an
independent resolution. It is not necessary to store the data set
prior to the analysis, although it may be convenient to do so. It
is certainly unnecessary to store the value of the energy at
unwanted frequencies.
EXAMPLE
A data set contains energy at frequencies g1, g2 and g3. g1 and g2
are separated by 0.01 Hz. g3 is separated from g1 and g2 by 10 Hz.
Assume that it requires a data sample of at least 100 seconds to
resolve g1 and g2, and that g3 can be adequately analyzed by a
sample of 0.1 sec. Assume further that the sample rate, which
should be at least twice the highest frequency of interest, is 100
KHz, and that a single sample is represented by 8 bits (1 byte).
The data storage required to store the data samples would be 10
Mbytes if the FFT were to be performed. In the case of the DFT, no
storage of the data is needed, because the Fourier analyses can be
performed in real time as the data is being collected in the mass
spectrometer. Such real time analysis is not possible with he FFT,
because analysis cannot proceed until the all of the data set is
available.
Using the DFT g1, g2 and g3 can be analyzed with different
resolutions. The data obtained from the spectrometer is simply a
string of data appended during multiple sampling intervals that are
established by the 100 kHz sampling rate. A data reading is
obtained that is long enough to secure the highest resolution
desired among the frequencies of interest. For frequencies at which
a lower resolution will suffice it is only necessary to utilize a
portion of sampling intervals in the DFT computation that comprise
the data. In performing the DFT a portion of the digitized output
waveform in the instrument is associated with a digitized reference
waveform for each frequency to be analyzed. Further savings of
computational resources can be thus be achieved.
As should be evident from the disclosure above, the apparatus of
the present invention can be programmed to analyze only those
frequencies of interest to the exclusion of all other frequencies
within the range of the instrument. Known background gases, such as
nitrogen and argon can be used to simply and accurately calibrate
the instrument. From this information exact frequencies relating to
specific events can be digitally established. Entire segments of
the frequency or spectral range of the instrument can be excluded
from the analysis because of prior knowledge developed by user or
simply because this data lies in a domain in which there is no
specific interest. The amount of computer power and time devoted to
resolving signal can thus be concentrated on those discrete
frequencies of interest which typically lie in a limited number of
bands within the spectral range of the instrument. Accordingly,
economy of both time and money is realized while increasing data
resolution.
The present system is capable of operating with low resolution A/D
converters while still providing high resolution processing of the
frequencies of interest. Furthermore, the system can be programmed
to analyze only specific frequencies of interest as well as the
bandwidth resolution required to properly analyze these signals.
Additionally, the control program for operating the systems and to
interface the various circuits involves a straightforward
application of conventional programming procedures which can be
accomplished by an experienced programmer.
While this invention has been explained with reference to the
structure disclosed herein, it is not confined to the details as
set forth and this application is intended to cover any
modifications and changes as may come within the scope of the
following claims.
* * * * *